1. Field of the Invention
The present invention relates to an electronic endoscope for observing an internal organ, especially, it relates to a color-adjustment process for image signals read from an image sensor.
2. Description of the Related Art
A lamp for illuminating a subject is provided in an electronic endoscope. Light emitted from the lamp passes through a fiber-optic bundle provided in a video-scope, and is directed to the distal end of the video-scope. The light emitted from the distal end of the video-scope is reflected on a subject, thus a subject image is formed on an image sensor (for example, CCD (Charge-Coupled Device)) provided at the distal end of the video-scope.
Generally, to display a color subject image on a monitor, an R, G, B sequential system using a rotating color filter or an on-chip color filter method using an on-chip color filter is used in the imaging process. Thus, image signals corresponding to the primary colors (R, G, B) or image signals corresponding to the complementary colors are generated and then read from the image sensor. A color adjustment process including a white balance adjustment, and so on, is performed to the read image signals, so that the subject image is reproduced on the monitor with proper color.
For the lamp, generally, a xenon lamp, a metal halide lamp, or a halogen lamp is used, and each lamp has a particular spectrum distribution. For example, light emitted from the halogen lamp includes a lot of light-components with wavelength corresponding to the color yellow. Consequently, the light with a color close to yellow color is emitted. On the other hand, the xenon lamp emits light close to daylight. Accordingly, color signal components in the image signals vary with different spectrum distributions, in other words, the difference of the type of lamp. To adjust the color balance of the subject image displayed on a monitor, a color adjustment process is performed in accordance with the spectrum distribution.
However, when a signal processing circuit is manufactured in accordance with a particular lamp, the lamp cannot be exchanged for an other type of lamp without modifying the signal processing circuit. When using other types of lamps, an improper color adjustment process is performed, so that color in the subject image cannot be properly reproduced. Accordingly, an electronic endoscope should be designed and manufactured for each type of lamp.